1. Field of the Invention
This invention relates to the design of a hard disk drive (HDD) suspension to support a slider mounted read/write transducer. More particularly, it relates to the design of a gimbal that provides superior dynamic performance during drive operation.
2. Description of the Related Art
A hard disk drive (HDD) uses an encapsulated thin film magnetic read/write head (transducer), called a slider, to read and write data on a magnetic medium or storage disk. The slider has a pre-patterned air-bearing surface (ABS) and is mounted on a flexible head gimbal assembly (HGA) that is itself mounted on a loadbeam. The combination of the loadbeam, the gimbal assembly (also referred to as a flexure), electrically conducting leads (or traces) that are routed along the gimbal and connect the slider to external circuitry (typically a pre-amplifier), a hinge mechanism and a baseplate, is collectively termed the suspension. The suspension is activated by a servo actuator and associated electronic control circuitry to position the slider at various target locations along the magnetically encoded tracks on the disk. As the disk is rapidly rotated by a spindle motor, hydrodynamic pressure causes an air flow between the ABS of the slider and the surface of the disk. This flow, called the air-bearing layer, lifts and suspends the slider so that it literally flies above the surface of the disk (at a “fly height” of approximately 10 nm) on a layer of air called, appropriately, the air-bearing layer. The edge of the slider into which the disk rotates is called its “leading edge,” the opposite edge, which contains the read/write head is called the “trailing edge.” The loadbeam, as is known in the art, has a small protrusion or “dimple” formed on its disk-facing side that presses against the backside of the slider, providing a downward force and a pivot point for the slider to rotate about. This suspension system of loadbeam and gimbal provides mechanical support for the slider while also allowing the slider pitch and roll capability when fly height is achieved. In addition, the system provides an electrical connection (i.e., a placement for the routing of conducting traces) between solder connections on the slider (connecting to the read/write head) and the pre-amplifier.
Enabling the slider to fly in a stable manner above the disk places stringent requirements on the suspension design, such as providing a proper range of its vertical stiffness (Kz), gimbal pitch and roll stiffness (Kp, Kr), gimbal pitch/roll static attitude (PSA/RSA), operational shock performance (G/gram) and the like. These requirements are mainly static and based on system geometry. A further requirement is that the suspension have little or no dynamic effect on the air bearing performance of the slider when the drive is operational. This requirement is related to the dynamic performance of the suspension.
In general, dynamic performance requirements are not always enforced, so the dynamic performance of many existing suspensions is not good. The role of the gimbal design in improving suspension dynamic performance merits study. In this regard, there are elements of the gimbal structure that are meant to provide improved static performance of the suspension system, but their design can have an adverse impact on the dynamic performance of the suspension. For example, the structure of the gimbal includes a ramp limiter that is meant to protect the slider from shocks when the drive is not operating and the slider is “parked”, but it is found that the design of the limiter has an effect of the manner in which the slider rides on its air bearing layer. In addition, the gimbal is designed with a view to accommodating the electrically conducting traces that connect the solder ball bonding (SBB) terminals of the read/write transducer to the external circuitry of the HDD. It is discovered, however, that the manner in which the traces are laid out and the route along which they are laid out also affects the dynamic performance of the suspension.
FIG. 1 and FIG. 2 shows the effect of suspension dynamics on the vibrational modes of a slider. FIG. 1 is a graphical representation of the vibrational modes of a slider that is mounted on a massless suspension, subsequent to a head-disk interface (HDI) interaction, where the slider strikes a surface asperity (a bump) on a rotating disk or where the slider is affected by lubricant on the disk surface. As can be seen, the interaction produces two damped vibrational modes, at approximately 105 kHz (1) and 315 kHz (2). The damping is inferred from the gentle rise and fall of the curves as well as their width.
FIG. 2 shows what is essentially the same slider, now mounted on a normal suspension, undergoing a similar HDI interaction. As can be seen, there is now a plurality of vibrational modes that are excited and, from their sharpness and height, it is clear that they are poorly damped. Those modes at low frequencies (eg. between 20 and 100 kHz) that are not well damped make it difficult for the slider to relax to an equilibrium condition. They can even cause the air bearing condition to become unstable and generate a sustained vibration of the slider. Under such circumstances the disk drive will fail or, at the least, its performance will be seriously degraded.
FIG. 3a to FIG. 3d show, schematically, four exemplary prior art gimbal designs illustrated as viewed from different perspectives so that important design features can be seen. FIG. 3e-FIG. 3f are typical vibrational response curves of sliders that are mounted on any one of such prior art gimbals.
FIG. 3a and FIG. 3b are shown from an overhead perspective (looking downward towards the disk surface), so the slider (not shown) would be mounted on the mounting pad (60), on the underside of the gimbal. FIG. 3c and FIG. 3d are shown from a perspective looking upward from the disk surface, so the slider (100) can be seen in FIG. 3c and the underside of a slider mounting pad (60), without a mounted slider (for clarity), can be seen in FIG. 3d. Symmetrically placed electrical traces (30) are shown in each figure with various routings from the distal end of the gimbal (top of figure) to the proximal end (bottom of figure). It is to be noted that the routing of the traces is such that the traces overlap with and are in contact with substantial portions of the stainless steel structure of the gimbal itself. This is particularly true in the encircled area (15) indicated in each of FIG. 3a-FIG. 3d. 
Looking more closely at FIG. 3a, there is seen the slider mounting pad (60) from an overhead view. Extending distally from the pad there are seen two arms (20) that comprise the ramp limiter for this design (note that the ramp limiter need not be formed in two pieces as in this design). A pair of stiffened traces (30) can be seen extending along the lateral sides of the gimbal. The encircled area (15) indicates where the traces extend laterally outward to reach the gimbal outriggers (40), which are the flexing portions of the gimbal, and, in doing so, the traces overlap substantial portions of the gimbal structure. In this region they are, therefore, in substantial contact with the stainless steel structure of the gimbal, although the traces are insulated from any electrical contact with the gimbal.
Although the traces are insulated and stiffened by an overlay of stainless steel (not shown), they are supported by tabs (50) that extend inward from the gimbal outriggers (40). An opening in the mounting pad exposes the terminal ends (35) of the traces at the position where they would be connected to the slider's solder bonding balls (SBB).
FIG. 3b shows a similar gimbal design wherein the traces (30) now extend laterally outside the outriggers (40). Once again, the encircled region (15) indicates where the traces are in substantial contact with the gimbal structure.
FIG. 3c shows the underside of a gimbal design (with a slider mounted thereon) in which the ramp limiter (20) is not formed in two separated pieces. Like the design of FIG. 3a, the traces (30) are routed over a substantial portion of the gimbal (15) then pass inside of the gimbal outriggers (40) and are not supported by tabs because they are stiffened. The terminal ends (35) of the traces are shown connected to solder ball (SBB) terminals on a slider (100).
FIG. 3d is another underside view of a gimbal design (with no mounted slider) that, like the design of FIG. 3c has a single ramp limiter (20). The traces (30) are supported by tabs (50) extending inward from the outriggers (40). As in the previous figures, the encircled region (15) shows the substantial contact between the traces and the gimbal.
Typical low frequency vibrational slider modes excited when sliders mounted on the gimbals of FIGS. 3a-3d engage in HDI interactions are shown in FIG. 3e-FIG. 3f. As is clearly shown, the modes are narrow and sharp, indicating very little damping, and the fact that they are at low frequencies (below 100 kHz) is evidence that more damaging effects may occur.
Gimbal design has received some attention in the prior art. Pan et al. (U.S. Pat. No. 6,965,501) discloses a gimbal design with a limiter that has a single arm. The design of the limiter allows easier bonding of the traces to the slider, it uses less insulation and it permits the loadbeam to have a narrower tip. The gimbal design does not address problems of suspension dynamics, however.
Danielson et. al (U.S. Pat. No. 6,667,856) discloses a gimbal design with an additional shock absorbing mechanism. This mechanism is meant to absorb shocks from excessive gimbal excursions due to encounters between the slider and the disk. It is not a mechanism that is meant to prevent such shocks from occurring.
Albrecht (U.S. Pat. No. 6,226,154) discloses a gimbal for a load/unload ramp having an improved system for parking the sliders. This design does not address the problem of gimbal dynamics during operation of the HDD.
It is clear from a reading of the prior art cited above that there is a need to improve slider response to vibrational motion produced by head-disk interface (HDI) interaction of a loadbeam mounted slider with disk asperities (i.e., bumps, lubricant on the disk and other deviations from disk planarity) while the disk is in rotational motion.